Generally, an azimuth resolution of detecting devices (e.g., radar apparatuses) is dependent on a beam width, and the beam width is defined by an aperture length of an antenna. Specifically, the azimuth resolution is defined by the aperture length of the antenna, and the azimuth resolution can be improved by extending the aperture length of the antenna. However, if the aperture length of the antenna is extended to improve the azimuth resolution, the size of the radar apparatus will become large. Therefore, it is requested to improve the azimuth resolution without extending the aperture length of the antenna. Also a range resolution (temporal resolution) of radar apparatuses is defined by a frequency bandwidth of the system and can be improved by extending the frequency bandwidth of the system. However, extending the frequency bandwidth of the system is technically difficult and also leads to a cost increase; therefore, it is requested to improve the range resolution without extending the frequency bandwidth.
To improve the azimuth resolution without extending the aperture length of the antenna and improve the range resolution without extending the frequency bandwidth, a method is known to perform inverse filtering on a reception signal. For example, JP3160580B discloses an art for improving the azimuth resolution by performing the inverse filtering. Specifically, first, a reception radio wave received by an antenna is converted into a reception electric field signal by a reception circuit, and the electric field signal is Fourier-transformed by a Fourier transformer. Next, the Fourier-transformed electric field signal is divided by a Fourier-transformed antenna pattern. After such inverse filtering, inverse Fourier transform is performed on the electric field signal, and this signal is outputted. By performing the inverse filtering as above, the azimuth resolution can be improved. The range resolution can also be improved in a similar manner.
Moreover, as the processing performed after the inverse filtering, other methods using so-called super-resolution methods, such as the Capon method, the MUSIC method and the Prony method, are also proposed instead of the inverse Fourier transform, so as to improve the azimuth resolution or the range resolution even more (see JP3032186B, JP2010-183979A, JP3160581B, “Study of Multi-path Propagation by Antenna Pattern Analysis” by Keiichi Sakurai, Kazuaki Takao and Iwane Kimura in The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report (Antennas and Propagation), Vol. 77, No. 101, pp. 1-6 (1977), “Direction-of-Arrival Estimation of Indoor Multipath Waves by Rotatory Scanning of Antenna using MUSIC algorithm” by Makoto Anzai, Masaru Ogawa, Koichi Yamada, Nobuyoshi Kikuma and Naoki Inagaki in IEICE Technical Report (Electromagnetic Compatibility), Vol. 89, No. 349, pp. 7-12 (1989), Chapter 13 of “Adaptive Signal Processing by Array Antenna” by Nobuyoshi Kikuma published by Kagaku Gijutsu Shuppan (2004), and “Experimental Study of High-Range-Resolution Medical Acoustic Imaging for Multiple Target Detection by Frequency Domain Interferometry” by Tomoki Kimura, Hirofumi Taki, Takuya Sakamoto and Toni Sato in Japanese Journal of Applied Physics 48 (2009)).
Furthermore, “High Range Resolution Ultrasonographic Vascular Imaging Using Frequency Domain Interferometry with the Capon Method” (IEEE Transactions on medical imaging, Vol. 31, No. 2, February 2012) discloses an art for reducing a false image generated in an echo image due to using any of the super-resolution methods described above, by using a moving average.
However, if the false image is reduced by using the moving average as described above, a part of the echo image where the false image is not generated may also be influenced by the false image.